The need for computer security is clearly evident from numerous articles in many publications identifying problems of data misappropriation, destruction, etc. The problem of providing a simple expedient to deter or prevent unauthorized access to computer drives for the purpose of data theft, introduction of computer viruses to the system, etc., has not been subject to resolution, especially in the original equipment markets. Applicant's patent, U.S. Pat. No. 4,685,312 issued Aug. 11, 1987 addresses aspects of computer security problems and describes retrofit mountable or portable devices to solve those problems. Since the descriptions in that patent are relevant to certain aspects of this invention, the content of that patent is incorporated by reference herein.